This invention relates to a discharge tube which is mainly used for various analyses or quantitative measurements.
One example of a discharge tube of this type is a deuterium lamp as shown in FIGS. 2(a) through 2(c). The lamp has first, second, third and fourth pins 1, 2, 3 and 4 to support electrodes. Those pins are arranged in a line at predetermined intervals in a relatively flat pinched stem 5 of glass. The fourth pin 4 supports a shielding electrode 6 directly. The third pin 3 supports an anode 7. The second pin holds the negative (-) side of a cathode 9 through a ceramic pipe 8, while the first pin 1 holds the positive (+) side of the cathode 9. The shielding electrode 6 has an electron converging part 11 including a small hole confronting the anode 7. The ceramic pipes 8 and 10 are fixedly secured to the shielding electrode 6 with bands 12. The ceramic pipe 10 is connected to the first pin 1. In FIG. 2, reference numeral 13 designates a sealed envelope of glass.
These electrode holding pins 1 through 4 generally have a diameter (d) in the range of: EQU 0.8&lt;d &lt;1.2 mm.
In this case, it is necessary that the distance L.sub.g between the junction of the fourth pin 4 and the shielding electrode 6 and the outer end of the pinched stem 5 is more than 12 mm, the minimum distance L.sub.a between the electron converging part 11 and the outer end of the stem 5 is more than 24 mm, and the distance L.sub.k curved along the pin from the electron emitting center of the cathode 9 to the outer end of the stem 5 is more than 28 mm; otherwise heat generated in the lamp when turned on (such as heat generated by the impact of hydrogen ions on the hot-cathode surface, heat generated in the surface of the electron converging part 11 by the emission of light at the electron converging part 11, and heat generated when electrons strike the anode 7) would cause thermal adverse effects on the glass system of the lamp. In the pinched stem 5, the glass and the metal material (of the electrode holding pins) are fused with each other which are different in thermal expansion coefficient from each other. Therefore, for instance when the lamp input is 30 W, if the above-described conditions L.sub.g &gt;12 mm, L.sub.a &gt;24 mm and L.sub.k &gt;28 are not satisfied, cracks may occur in the glass stem 5.